What phenomenon makes a changing electric field produce a magnetic field, completing Maxwell’s equations?

Displacement current is the effect that makes a changing electric field produce a magnetic field in Maxwell’s equations.

James Clerk Maxwell introduced the concept in the 1860s. Ordinary conduction current involves moving electric charge through a material, but a changing electric field can also contribute to the magnetic field. This contribution is especially important in the gap between the plates of a charging capacitor, where charge does not cross the dielectric yet the electric field changes.

Maxwell added the displacement-current term to Ampère’s circuital law. The correction made the equations consistent with charge conservation and predicted that coupled changing electric and magnetic fields could travel through space as electromagnetic waves.

The term does not necessarily mean that particles physically cross a capacitor gap. It is a field contribution with the same magnetic effect in the relevant equation. Light, radio waves, microwaves, and X-rays are all electromagnetic radiation described by Maxwell’s theory.